General Rules in Multithreading

Today I’ve read this very good blog post about using multithreading in Qt, and even if today I don’t manage threads by myself, that reminded me my first attempt in understanding and explaining the multithread.

My first attempt in doing multithread was while studying (by my own) Java. It was Java 1.1, and the language included a very simple interface to multithreading, so it was quite simple to launch an applet that spawn a few threads to do some flash GUI updates.
Later I discovered other Java system were, of course, using multithreading. And while I had to use, in my university, other multithreaded C libraries, my main focus on multithreading was in Java.

Back in those days, multithreading seemed to me really easy: you create a new thread, synchronized other a shared mutex, and finalize the thread once it has done its job.

So far, so good!

But then came the GUI development.
Of course, a GUI application as a few threads running for you and handling all the small details. But when you are addicted to spawn-synchronize multiple threads you tend to do the same even in GUI applications, and you application freezes!
It is obvious, once you understand it: a GUI application is a thread-based application, but it does synchronization by message passing. Some languages calls it events, other event loop, and so on. The idea is simple: your main thread is processing stuff out of an event queue.
The rule of thumb of the above is that you do not block the event thread or all your application will freeze.
With “blocking” I mean either put to sleep or let it do a very intensive task (e.g., updating a lot of rows on a database connection, download a file, and so on).

I learnt the above on my own experience, luckily while I was still a little student at university, but I have to confess that I made the same mistake on my day-to-day job too! Therefore, I was really glad to see the blog post placed the same advice as the first items on the list: 1) Never call QThread::sleep() 2) Never do GUI operations off the main thread 3) Don’t block the main thread

Of course, the above advices does not apply only on GUI applications, but anywhere you have an event-loop based mechanism. In fact, I developed the suspend() method in Aglets to emulate the Thread.sleep() method, making an agent something more similar to a thread.